* linux-low.h (struct linux_target_ops): Replace left_pad_xfer field

	* linux-low.h (struct linux_target_ops): Replace left_pad_xfer field
	by collect_ptrace_register and supply_ptrace_register hooks.
	* linux-low.c (fetch_register): Use supply_ptrace_register callback
	instead of checking for the_low_target.left_pad_xfer.
	(usr_store_inferior_registers): Use collect_ptrace_register callback
	instead of checking for the_low_target.left_pad_xfer.

	* linux-s390-low.c (s390_collect_ptrace_register): New function.
	(s390_supply_ptrace_register): Likewise.
	(s390_fill_gregset): Call s390_collect_ptrace_register.
	(the_low_target): Update.

	* linux-ppc64-low.c (ppc_collect_ptrace_register): New function.
	(ppc_supply_ptrace_register): Likewise.
	(the_low_target): Update.

	* linux-i386-low.c (the_low_target): Update.
	* linux-x86-64-low.c (the_low_target): Update.